1. Field of the Invention
This invention relates to a plastic snap hinge closure, having a first part to be fastened on a container and a second part, which is connected in one piece with the first part by a main film hinge, and wherein at least one spring element engages both parts and bridges the axis of rotation of the main film hinge. This invention also relates to a process for manufacturing such a plastic snap hinge closure.
2. Description of Prior Art
European Patent Publications EP-B-0 141 591 and EP-A-0 447 357 and U.S. Pat. No. 4,414,705 provide examples of such closures. The stability and durability of a plastic closure is essentially determined by the design of the hinge. A problem arises with snap hinge closures since the main film hinge connecting the two hingedly connected closure parts must often be relatively short in length and must be interrupted at least in the area of the spring elements, which generate a snap effect. Examples of the design of such snap hinge closures are described in the above-mentioned patent references.
In all of these conventional snap hinge closures, the main film hinge is interrupted by a spring element which is centered between the two parts, so that it is reduced to the two parts of the main film hinge remaining on either side of the spring element. In such closures, with the exception of the embodiment taught by European Patent Publication EP-A-0 447 357, the main film hinge or portions of the main film hinge are offset towards the outside, relative to the circumference of the closure. Besides the lack of a pleasing esthetic appearance, this results in a considerable reduction in the length of the main film hinge and also results in a weakening of the hinge and thus of the entire plastic closure.
Other conventional plastic closures, which were first marketed by Applicant, have two or more spring elements that act as tightening straps. This type of plastic closures avoids the incisions otherwise required on both sides of the spring elements, and the main film hinge is not interrupted. However, these closures were also unable to satisfy the esthetic requirements of the marketplace. Added to this was the fact that the spring element protruded beyond the circumference of the closure and certain problems occurred in connection with automatic filling and closing devices. These difficulties are avoided by a further development of the plastic closure in accordance with European Patent Publication EP-B-0 291 457. Here, the spring elements on either side of the main film hinge are relocated into recesses in the casing walls of the upper and lower part of the plastic closure. But here, too, the spring elements in the form of trapezoidal tightening straps are clearly visible from the exterior of the closure.
The length of the main film hinge is also again limited to the distance between the two spring elements. Something similar is true for the plastic closures in accordance with European Patent Disclosure EP-B-0 056 469. Such spring elements are either spiral springs or tension elements and extend with their auxiliary film hinges into an extremely short main film hinge. This results in an imprecise closing operation and also reduces the stability of the hinge.